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Methanol and formic acid oxidising anodes in zinc electrowinning
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
2006 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

Electrowinning is an energy demanding process, in which the electrolysis accounts for the major part of the energy consumption. The anodic reaction in the electrolysis of zinc from sulphate based electrolytes is oxygen evolution, which has a standard potential of 1.23 V at 25 ºC. The lead anodes used in zinc electrowinning today have a high overpotential for oxygen evolution and the operating anode potential is as high as 2 V vs NHE. Since the lead anodes are not stable some of the lead can dissolve and incorporate in the zinc metal on the cathode. The lead anodes are also undesired because of health concerns. By introducing formic acid or methanol oxidation as anode reaction, the anode potential could be lowered significantly. This would reduce the energy consumption and also enable the use of new types of electrodes.

The general aim of this thesis was to investigate how depolarised anodes for methanol and formic acid oxidation can be used in electrowinning processes.

Several electrodes were shown to be suitable as anodes in zinc electrowinning. A high activity was obtained with porous electrodes, whereas a platinum coated titanium was the most stable electrode. The choice of electrode will however always be a compromise between activity, stability and costs. All electrodes were deactivated with time and the best method for reactivation tested in this study, was periodic current reversal (PCR).

An operating procedure was established for methanol and formic acid oxidation on high surface area electrodes, Pt-TySAR®. With the use of PCR at process current density, a potential lower than 0.7 V vs NHE could be maintained for 36 hour in synthetic electrolyte. The use of formic acid oxidation resulted in lower potentials than the use of methanol oxidation.

The activities for methanol and formic acid oxidation in industrial electrolyte were very low and chloride impurities were shown to cause the major part of the activity reduction. Even small amounts of chloride (10-6-10-5 M) affected the activity of the electrodes. The chloride impurities are considered as the main problem to overcome in order to introduce the oxidation of formic acid or methanol in the zinc electrowinning process.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , 36 p.
Series
Trita-KET, ISSN 1104-3466 ; 230
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4201OAI: oai:DiVA.org:kth-4201DiVA: diva2:11211
Presentation
2006-11-28, Salongen, KTHB, Osquars Backe 31, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20101004Available from: 2006-12-01 Created: 2006-12-01 Last updated: 2010-10-04Bibliographically approved
List of papers
1. Methanol oxidation as anode reaction in zinc electrowinning
Open this publication in new window or tab >>Methanol oxidation as anode reaction in zinc electrowinning
2005 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 152, no 11, D201-D207 p.Article in journal (Refereed) Published
Abstract [en]

In this study, different types of Pt- and PtRu-based electrodes were compared regarding their activity and stability for methanol oxidation in an acid zinc sulfate solution. The lead anodes used in zinc electrowinning today are not dimensionally stable and have a high overvoltage for oxygen evolution. By replacing the oxygen evolution in sulfate-based electrolytes with methanol oxidation, the anode potential could be significantly lowered. This would reduce the energy consumption and also enable the use of new and more stable types of electrodes. The activities of the electrodes studied were found sufficient for electrowinning. The highest activity was obtained with porous PtRu, while platinized titanium was the most stable electrode. All electrodes were deactivated with time, and the deactivation rate of PtRu was influenced by mass transport. The dominating reaction mechanism seemed to be different on Pt and on PtRu. Due to deactivation of the electrodes with time, a reactivation method was needed. The best method tested in this study was to periodically reverse the current. After assessing the electrodes with respect to important properties, the platinized titanium electrode was considered to be most suitable for use in electrowinning processes.

National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-24976 (URN)10.1149/1.2039567 (DOI)000233133700044 ()2-s2.0-27944434717 (Scopus ID)
Note
QC 20101004Available from: 2010-10-04 Created: 2010-10-04 Last updated: 2017-12-12Bibliographically approved
2. Methanol and formic acid oxidation in zinc electrowinning under process conditions
Open this publication in new window or tab >>Methanol and formic acid oxidation in zinc electrowinning under process conditions
2008 (English)In: Journal of Applied Electrochemistry, ISSN 0021-891X, E-ISSN 1572-8838, Vol. 38, no 1, 17-24 p.Article in journal (Refereed) Published
Abstract [en]

The possibility of using methanol or formic acid oxidation as the anode process in zinc electrowinning was examined. The activity for methanol and formic acid oxidation on Pt coated high surface area electrodes was investigated over 36 h, at a current density used in industry. The activity could be maintained at a constant potential level in a synthetic electrowinning electrolyte if the current was reversed for short periods. During the tests, the anode potential was, more than 1.2 V below the potential for the oxygen evolving lead anodes used in modern zinc electrowinning. The lowered anode potential would lead to a significant energy reduction. However, tests in industrial electrolyte resulted in a very low activity for both methanol and formic acid oxidation. The low activity was shown to be caused mainly by chloride impurities. A reduction of the chloride content below 10(-5) M is needed in order to obtain sufficient activity for methanol oxidation on Pt for use in zinc electrowinning. Pt and PtRu electrodes were compared regarding their activity for methanol oxidation and the latter was shown to be more affected by chloride impurities. However, at a potential of 0.7 V vs NHE, with a chloride content of 10(-4) M, formic acid oxidation on PtRu gives the highest current density.

Keyword
anode reaction, zinc electrowinning, formic acid, methanol
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-24975 (URN)10.1007/s10800-007-9387-0 (DOI)000251370900003 ()2-s2.0-36749007865 (Scopus ID)
Note
QC 20101004Available from: 2010-10-04 Created: 2010-10-04 Last updated: 2017-12-12Bibliographically approved

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